Photosensory transduction in a microorganism, Phycomyces, will be investigated with modern methods of systems physiology. The Wiener theory of nonlinear systems identification will be applied to the transient light growth response of the sporangiophore. Wild type and representative photomutant strains will be studied comparatively. Experiments will be performed on the Phycomyces tracking machine under the control of a new microcomputer system. The light intensity will be modulated with two types of test stimuli, one stochastic and the other deterministic. The former, gaussian white noise, is standard in applications of the Wiener theory. The other, a sum of sinusoids, was recently developed elsewhere and shown to be superior in applications to a neural system. Present work on the white noise method will be completed as the new sum-of-sinusoids method is being implemented. A promising trial of the latter method has been performed recently on Phycomyces with the help of the microcomputer. The results, in the form of Wiener kernels or corresponding frequency kernels, will be interpreted in the framework of linear and nonlinear system analysis. The sensory transduction chain will be probed by means of single and double mutants with reduced phototropism as well as hypertropic mutants. The fundamental relationship between the light growth response and phototropism will be investigated theoretically and experimentally. A phototropism model, based on the cellular rotation and cylindrical optics of the sporangiophore will be elaborated and tested experimentally. A related kinetic model for the processes of light and dark adaptation will be refined and tested. The sporangiophore of Phycomyces serves as a model for the processes of sensory transduction and adaptation in primary receptor cells. The systems approaches employed are of increasing importance in the fields of sensory physiology and biomedical engineering.